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INTRODUCTION: Histophilus somni (formerly known as Haemophilus somnus) was first The Authors were involved when an outbreak occurred one year ago in identified as a bovine pathogen responsible for thrombotic meningoencephalitis (TME) only 55 only young male “Marchigiana” cattle, characterised mainly by eye and years ago. The only known habitats of H. somni are the mucosal surfaces of ruminants, making respiratory disease that led to animal death. this bacterium an opportunistic pathogen. Nowadays, H. somni is responsible for a wide variety of systemic diseases in addition to TME, including respiratory disease syndromes, myocarditis, reproductive disease syndromes, polyarthritis, mastitis, eye disease and septicaemia. METHODS: Between January and April 2016, nine one-year-old male calves suffered from progressive eye disease, resulting in blindness, ataxia, weight loss and mild respiratory disease. All of these animals died within 20 days of the onset of symptoms. Nasal and eye swabs were collected from two others calves for bacteriological and virological analysis. After slaughter, the brain and testicles were also collected for macroscopic and histopathological examination. RESULTS: All the samples collected were negative for BoHV-1, BVDV, PI-3 and Mycoplasma spp., while Histophilus somni was isolated from the testicles, brain and eye swabs. Mannheimia haemolytica was also isolated from nasal swabs. H. somni was identified biochemically and confirmed by PCR. The isolated strains were tested for antibiotic resistance (Table). Mm of inhibition Result Mm of inhibition Result Antibiotics Histophilus Histophilus Mannheimia Mannheimia Amikacin 17 Intermediate 19 Sensitive Amoxicillin + Clavulanic Ac. 0 Resistant 10 Resistant Cefadroxil 18 Sensitive 19 Sensitive Cefquinome 24 Sensitive 22 Sensitive Enrofloxacin 22 Sensitive 23 Sensitive Gentamicin 16 Intermediate 19 Sensitive Neomycin 17 Sensitive 18 Sensitive 1A 1B Doxycycline 10 Resistant 0 Resistant Oxytetracycline 12 Resistant 0 Resistant The most constant gross neurological changes observed in these animals Cefalexin 18 Sensitive 19 Sensitive were moderate congestion of meningeal vessels, and foci of haemorrhage in Tylosin 0 Resistant 10 Resistant the cerebral cortex (Fig. 1A). Macroscopic lesions that were observed in testicles and epididymis were: presence of adherence sites between the scrotum and dartos fascia, unilateral or bilateral increase in size and presence of pyogranulomatous nodules at incision with mild purulent discharge (Fig. 1B). Histologically, the epididymis tail sample exhibited abundant disorganized fibrous connective tissue and cystic formations (dilated ducts) of different sizes composed of stratified squamous epithelium (epidermoid metaplasia) surrounded by blood vessels containing large numbers of inflammatory cells (Fig. 2A). These cells were predominantly neutrophils and to a lesser extent lymphocytes, monocytes and macrophages, together with abundant hyaline proteinaceous material (Fig. 2A, insert). Thrombi occurred predominantly within small vessels of the meninges and in all regions of the brain (Fig. 2B- 2A 2B D) of all the animals investigated. The thrombi consisted of discrete intravascular accumulations of neutrophils and/or lymphocytes admixed with fibrin and moderate to marked proliferation of the affected vascular endothelium (Fig. 2D). In addition, varying degrees of cortical oedema, gliosis, malacia and necrosis were observed in the evaluated brain sections (Fig. 2B). Perivascular cuffing of admixed neutrophils, lymphocytes, monocytes, and macrophages were also observed frequently in different brain regions (Fig. 2C). Other tissues did not exhibit evidence of specific pathological changes. CONCLUSIONS: Specific techniques are required for isolation of H. somni, and an overgrowth of other bacteria can mask its presence. The use 2C 2D of PCR enables a higher percentage of positive animals to be detected, but this method does not allow antibiotic susceptibility to be performed. This All the remaining calves were injected with HIPRABOVIS® SOMNI/Lkt starting from the can lead to the use of ineffective antibiotics and thus an increase in second month of age. The second inoculation was performed after 21 days. After a follow-up of antibiotic resistance. 12 months, no other case of infection was diagnosed. This outbreak also confirms the presence of H. somni in Central Italy and confirms its responsibility for the onset of TME phenomena which are often lethal, especially in calves under one year of age. Effective prevention through vaccination can reduce the appearance of clinical signs, lung damage and consequently the use of antibiotics. References: Inzana T.J. Current Topics in Microbiology and Immunology: Histophilus somni Biology, Molecular Basis of Pathogenesis, and Host Immunity: Springer Ed. doi 10.1007/978-3-319-29556-5 Tegtmeier C, Angen O, Ahrens P. 2000. Comparison of bacterial cultivation, PCR, in situ hybridization and immunohistochemistry as tools for diagnosis of Haemophilus somnus pneumonia in cattle. Vet Microbiol. 76(4):385-94. Murray GM. et al. 2017. Pathogens, patterns of pneumonia, and epidemiologic risk factors associated with respiratory disease in recently weaned cattle in Ireland. J Vet Diagn Invest. 29(1):20-34. doi: 10.1177/1040638716674757.

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